1. Field of the Invention
The present invention relates to an operating method of a semiconductor device. More particularly, the present invention relates to an operating method of a multi-level cell (MLC) of a non-volatile memory, which utilizes a double-side bias band-to-band tunneling hot hole (DSB-BTBTHH) effect in programming.
2. Description of the Related Art
With the rapid growth of information flow in the world, the requirement for the storage capacity of electrically erasable programmable non-volatile memory devices, such as flash memory devices, continuously gets higher in the market.
The most direct way to increase the unit-area storage capacity of a non-volatile memory product is to reduce the lateral area of each memory cell, while this is limited by the lithographic resolution and some dimension-relating electrical properties, etc. Another way is to allow each memory cell to store more than one bit of data, which may be done by making a cell have one of 2n (n≧2) threshold voltages that correspond to 2n storage states. Such a memory cell is called a multi-level cell (MLC).
In conventional MLC operation, the erasing is done by ejecting electrons out of the charge-storing layer of the cell through Fowler-Nordheim (FN) tunneling, and the programming by injecting electrons into the same through FN tunneling. The amount of injected electrons is controlled by the programming time so that a cell has a threshold voltage (Vt) corresponding to the predetermined storage state thereof.
However, because the efficiency of injecting or ejecting electrons through FN tunneling is low, the erasing or programming is slow in the prior art. Moreover, since the erasing step ejects electrons out of the charge-storing layer, the threshold voltage of the cell is lower in the erased state causing more leakage. Further, the programming step that controls the threshold voltage in time mode cannot precisely control the amount of injected electrons, so that the Vt-distribution of each storage state is broad easily causing error in the reading.